The present invention relates to the transmission of digital data and, more particularly, to the transmission of digital data over partial-response channels, i.e., channels which introduce intersymbol interference.
The problem of transmitting data over partial-response channels arises in a number of commercial contexts. Among these contexts is the transmission of high-definition television (HDTV) signals in existing frequency bands that are allocated to standard (NTSC) TV signals. The problem specifically arises in areas where channels that are unused for NTSC transmission--and are thus candidates for HDTV transmission--are also in use for NTSC transmissions in relatively proximate areas. As an example, TV channel 3 is currently unused in the New York City metropolitan area, but is used in both Philadelphia and Hartford. The consequence of such proximity is that if channel 3 is to be used for HDTV in New York City, the HDTV signal may be corrupted by the channel 3 NTSC signal from Philadelphia or Hartford for New York City HDTV viewers. Conversely, the NTSC signal may be corrupted by the HDTV signal. Such corruption is referred to as "co-channel interference."
It has been realized that one can satisfactorily deal with the co-channel interference from the HDTV signal to the NTSC signal by specifying in the HDTV standards (still under consideration) a sufficiently low transmit power level. Moreover, in order to deal with co-channel interference from the NTSC signal to the HDTV signal, it has been proposed to rely on the fact that an NTSC signal is dominated by energy concentrations at particular locations in the frequency spectrum. Thus, it has been proposed to provide an HDTV receiver with a comb filter having nulls at those frequency locations, thereby removing a significant portion of the interfering NTSC signal, while only minimally degrading the HDTV signal.
The combination of the over-the-air TV channel with the comb filter constitutes a partial-response channel because the comb filter will create so-called "forced" or artificial intersymbol interference (ISI) in the received signal. (The term "channel" is used herein to mean either just the over-the-air broadcast channel or that channel in combination with various components of the receiver, as will be apparent from the context in each case.) Such ISI could be dealt with via the use of a decision feedback equalizer (DFE) in the receiver. However, the error propagation characteristics of DFEs render this a less-than-desirable solution for dealing with the forced ISI. Accordingly, it has alternatively been proposed that a precoder be provided in the HDTV transmitter so as to anticipate, and compensate for, the forced ISI, with the result that the received signal does not suffer from the forced ISI effect. A typical early such precoding technique is disclosed in P. Kabal and S. Pasupathy, "Partial-Response Signaling," IEEE Transactions on Communications, Vol. COM-23, No. 9, Sep. 1975, pp. 921-934.
There are two important aspects of any successful such precoding technique. One is that it must be carried out in such a way that the number of symbols of the transmit constellation is substantially the same as the number of different precoder input bit patterns. Although the forced ISI could be dealt with using a Tomlinson-filter-based precoder, such a precoder will, in general, violate this requirement, which arises out of considerations related to transmitted power limitations and receiver equalizer complexity. The other important aspect of a successful precoding technique is that the transmitted information bits be able to be recovered from a respective signal point of the comb-filtered signal, independent of the value of any other signal point thereof. This requirement arises out of the desire to avoid error propagation in the receiver.
To this point, the prior art has indeed been able to satisfy these requirements on the precoder, but only by limiting its proposed HDTV constellation designs to essentially two classes of constellations. One class is M-ary pulse amplitude modulation--a one-dimensional signaling scheme known as M-PAM with M being an integral power of 2. The other class is M.sup.2 -ary quadrature amplitude modulation, a two-dimensional signaling scheme known as M.sup.2 -QAM with, again, M being an integral power of 2. Such limitations are potentially problematic. Those skilled in the art appreciate that having the flexibility to select from a wider array of constellation types is advantageous in that, for example, it allows one to provide and trade-off among such system design considerations as a) providing a good match between bit rate and constellation size and b) peak-to-average power ratio. Moreover, I have realized that being able to flexibly select from a broad array of constellations allows one to more readily incorporate within systems using partial-response-channel precoding a multiplexing technique wherein the quality of a received HDTV (or other) signal degrades gracefully as a function of distance from the transmitter. Such technique is described generally (but not in conjunction with partial-response-channel precoding) in my co-pending U.S. patent application with H. Y. Chung and J. Wang, Ser. No. 07/627,156 filed on Dec. 13, 1990.
A further limitation of the prior art partial-response-channel precoding techniques is that they may not be readily usable with comb filters having other than a very simple transfer characteristic--a potential drawback if the application in fact requires a more complicated characteristic.